I also like Karin’s suggestion about reminding people that they can work at their own pace and that “challenging” is relative to the individual. I ran a very successful circuit class for 8 years, based upon that principle. Everyone set their own “hard.”
The other thing that helped me teach multi-level was getting to know my clients over time. My average class size was only 12-18 people, rotating between about 30 regulars and 50 sporadic attendees, so it was possible for me really know those 30 and their bodies well, and at least the key points of the other 50. I plan around the 30 (knowing not all will come to each class, but considering their needs), with lower an higher options.